Field of the Invention
The embodiments disclosed herein relate to a motor controller and a method for controlling a motor.
Discussion of the Background
Japanese Unexamined Patent Application Publication No. 2015-133878 discloses a motor controller that controls a motor. When an instantaneous power failure or a similar situation occurs, the motor controller temporarily keeps its inverter from supplying power to the motor, turning the motor into, for example, free-run state or coast-rotation state (these states will be hereinafter collectively referred to as free-run state).
After recovery from the instantaneous power failure, the motor controller causes the inverter to supply power to the motor. If there is a wide discrepancy between the rotational frequency of the motor and the output frequency of the inverter, such consequences may occur as: an undesirable amount of torque that may cause jerks; overcurrent; and overvoltage.
In order to avoid these consequences, the motor controller makes the rotational frequency of the motor and the output frequency of the inverter identical to each other when the inverter supplies power to the motor. For this purpose, the motor controller has a function to detect the rotational frequency of the motor in free-run state (this function will be hereinafter occasionally referred to as speed search).